The present invention relates generally to welding and, more particularly, relates to an improved apparatus for measuring the power output of a laser beam.
Laser welding is commonly used to join plastic or resinous parts, such as automobile thermoplastic parts, at a welding zone. An example of such use of lasers can be found in U.S. Pat. No. 4,636,609, which is expressly incorporated herein by reference.
As is well known, lasers provide a focused beam of electromagnetic radiation at a specified frequency (i.e., coherent monochromatic radiation). There are a number of types of lasers available; however, infrared lasers or non-coherent sources provide a relatively economical source of radiative energy for use in heating a welding zone. One particular example of infrared welding is known as Through-Transmission Infrared Welding (TTIR). TTIR welding employs an infrared laser capable of producing infrared radiation that is directed by fiber optics, waveguides, or light guides through a first plastic part and into a second plastic part. This first plastic part is often referred to as the transmissive piece, since it generally permits the laser beam from the laser to pass therethrough. However, the second plastic part is often referred to as absorptive piece, since this piece generally absorbs the radiative energy of the laser beam to produce heat in the welding zone. This heat in the welding zone causes the transmissive piece and the absorptive piece to be melted and thus welded together.
It is often important to have precise feedback information relating to laser intensity so as to adjust or at least measure the laser output during the welding process. This information must reliably indicate the true intensity of the laser beam to provide sufficient information to a controller or worker. However, because the entire weld zone is illuminated simultaneously in a TTIR plunge weld, it is impractical to use a conventional beam splitter to measure the laser intensity at the weld surface after exiting the fiber optic bundle. Additionally, because there are unpredictable coupling losses between the laser diode and the fiber optic bundle, it is unrepresentative and thus unreliable to measure the output light of the laser diode before it enters the fiber optic bundle. Therefore, any reliable sampling of the laser beam must be done while the laser beam passes through the fiber optic bundle.
Unfortunately, the distribution of laser light from the laser diode is not uniform. Therefore, inherently there is variation of the intensity of light in each individual optical fiber. Hence, the light intensity in a single individual optical fiber is not representative of the light intensity in the entire fiber optic bundle. Ideally, the desired feedback information provided by any sensor would be of the total population of fibers in a bundle, which would eliminate the variance that exists between individual fibers. Statistically, the larger the population of fibers sampled, the smaller the random variance is between the feedback signal (i.e. measure output) and the true power output. With reference to FIG. 4, the statistical relationship between the sample population and the variance between the measure output and the true output for a laser beam input whose uniformity varies by 50% is shown.
Past attempts to feed back a small percentage of the population of fibers to a photodiode were unsuccessful in part because the variance between the measure output and the true output power was too high. On the other hand, attempts to feed back a large percentage of the fibers yielded smaller variances, but consumed too much energy that could otherwise be used for welding.
Accordingly, there exists a need in the relevant art to provide an apparatus that is capable of minimizing the variance between the measure power output and the true power output of a laser. Furthermore, there exists a need in the relevant art to provide an apparatus that is capable of minimizing the variance between the measured power output and the true power output of a laser without consuming energy that could otherwise be used for welding. Still further, there exists a need in the relevant art to provide an apparatus and method of using the same that is capable of overcoming the disadvantages of the prior art.
According to the principles of the present invention, a radial power feedback sensor is provided that reliably and accurately senses the power output of a fiber optic bundle. The fiber optic bundle is arranged generally radially about an axis to carry radiative energy produced by a laser. A spacer is positioned within the fiber optic bundle such that the fiber optic bundle generally surrounds the spacer. The spacer serves to enable the radiative energy to pass therethrough. A photo detector is then disposed adjacent the spacer and is operable to output a signal in response to a measured intensity of the radiative energy passing through the spacer. Accordingly, due to the surrounding of the fiber optic bundle around the photo detector, a greater number of individual photo optic lines are exposed to the photo detector, thereby decreasing the variance between the measure output and the true output of the laser.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.